The problems involved in the design of effective air conditioning systems for mobile units are significantly greater than those involved in stationary systems. In general, the basic theory of operation of the stationary and mobile air conditioning units is the same. Each system requires a cyclic refrigerant flow through an evaporator to absorb heat from the space to be cooled and through a condenser to exhaust the absorbed heat. However in a stationary system, the compressor is usually driven at a constant speed, or in more efficient systems, at two or more selectable fixed speeds. The more efficient stationary systems may also have a selectable multi-speed blower for the evaporator. In any event in stationary systems the blower and compressor speeds are known controllable quantities. The uncontrollable variables in the stationary systems are primarily the ambient temperature of the air or coolant through the condenser and the temperature of the air flow through the evaporator.
In contrast, the mobile air conditioning systems (i.e. systems located on mobile vehicles such as in autos, trucks, buses, etc.) face the same temperature variables involved with the condenser and evaporator and concerning the air flow variables through the evaporator, but in addition include a variable speed compressor whose speed is a function of the engine speed, and an air flow through the condenser that is a function or the vehicle speed. All of these additional variables are controlled by instantaneous vehicle travel requirements, thereby greatly expanding the environmental and physical constraints placed on the effective operation on the vehicle air conditioning system. These added variables involved in mobile air conditioning systems involve two extreme situations, i.e., 1) idle when the vehicle is not moving and the engine is running at slow speed (low compressor speed and low condenser air flow), and 2) road run when both the engine and the vehicle are running at high speed (high compressor speed and high condenser air flow). The problem facing mobile air conditioning system designers are to develop systems that will perform satisfactory at both these extremes and in between.
An additional problem facing such designers particularly in the automobile industry is the lack of space and cost control. As automobile designs become more compact and greater demands are placed on fuel efficiency and pollution control, there are constant design pressures to reduce the size of elements in the air conditioning systems. In addition, as usual there is the on going need to contain or reduce cost. Competing with the constraints of space and cost, there is the continued need for improvements in quality of performance. Durability of design is also a very important factor so as to minimize failures, particularly those that are catastrophic in nature that result in the destruction of expensive elements such a compressor.
Presently, in mobile air conditioning systems of the type used in automobiles, the refrigerant flow to the evaporator is controlled either by a fixed orifice or a expansion valve. The object of is to attempt to achieve maximum performance by controlling the amount of refrigerant in liquid form as it exits the evaporator (i.e., a point at which most of the refrigerant tends to change from liquid to vapor). If total vaporization of the refrigerant takes place within the evaporator, a hot spot or section is created in the evaporator thereby reducing its effectiveness. Similarly, the excessive flow of liquid refrigerant from the evaporator also reduces the system performance.
A fixed orifice is an inexpensive means to control refrigerant flow to the evaporator, but suffers the defect that the size of the orifice must be selected as a compromise solution of performance between idle and road run. If the size of the orifice is selected to favor idle, then the system will perform favorably in city driving, but will suffer reduced performance in open road driving. In contrast, if the orifice size is selected to favor road run, then city driving suffers performance.
Some automobiles use a temperature sensing expansion valve to control the refrigerant flow through the evaporator as a primary function of the refrigerant temperature at the output of the evaporator. The temperature sensing mechanism in an expansion valve is inherently slow and therefor not responsive enough to the continually varying air conditioning demands of an automobile. Furthermore, temperature is a poor indication of the condition or state or the refrigerant as it leaves the evaporator in that it can only sense vapor flow and not liquid flow, providing only one half the equation. An additional problem with the use of the expansion valve is that it tends to fail in the closed condition, resulting in the shut off of refrigerant and lubricant flow to a level that causes destruction of the compressor.
It is an object of this invention to provide a new and improved air conditioning system for mobile air conditioning systems, and control mechanisms therefor, involving the control of refrigerant flow through the air conditioning system evaporator as a function of refrigerant pressure adjacent the evaporator.